WEDNESDAY, May 8 (HealthDay News) -- Exercise appears to change the way women's bodies metabolize the hormone estrogen, and this could help explain how aerobic activity reduces a woman's breast cancer risk, a new study suggests.

Previous research has suggested that exercise lowers breast cancer risk, but there haven't been any clinical studies that explain the mechanism behind this, said Mindy Kurzer, a professor in the department of food science and nutrition at the University of Minnesota in Saint Paul.

"Ours is the first study to show that aerobic exercise influences the way our bodies break down estrogens to produce more of the 'good' metabolites that lower breast cancer risk," she said.

The study included nearly 400 healthy, but inactive young women who were divided into two groups. All were premenopausal, meaning their bodies still produced the hormone estrogen.

One group remained inactive while the other group did 30 minutes of moderate-to-vigorous aerobic exercise five times a week for 16 weeks. For their workouts, they used equipment such as treadmills, stair steppers and elliptical machines.

Before and after those 16 weeks, the researchers collected urine samples from women in both groups. At the end of the study, the women in the exercise group had higher levels of the estrogen metabolites that reduce breast cancer risk.

The study was published May 7 in the journal Cancer Epidemiology, Biomarkers & Prevention.

"Exercise, known to favor fitness and improve heart health, is also likely to help prevent breast cancer by altering estrogen metabolism," Kurzer said in a news release from the American Association for Cancer Research. "It is very important, however, to decipher the biological mechanisms behind this phenomenon."

She is currently conducting similar studies in women at high risk for breast cancer.